This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Using LC-MS/MS, we have identified the target of a mechanism-based probe against cysteine proteases upregulated during apopotosis as Cathepsin B, implicating its activity in cell death.